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Leg Stiffness

Discussion in 'Biomechanics, Sports and Foot orthoses' started by mike weber, Mar 15, 2010.

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  1. You're are on the same page as me.
     
  2. and me.

    we can also adjust leg stiffness thru changing loading rates as well it seems from my reading. So we could contruct a device around the loading rates appart from material choice . ie ff striking has different loading rates to RF striking, So I assume that a medial skive device would have different loading rates to a Root device made from the same material.But I maybe wrong on that.

    I´m just not sure how.
     
  3. efuller

    efuller MVP

    With all this discussion about orthotics or an equinus changing stiffness we should think carefully about when in gait the stiffness is altered. Aren't most of these changes effecting stiffness at the instant of contact. After contact the stiffness is going to be dependent on the movement the person is attempting to do. Are they trying to fall on the floor, walk or jump?

    The magnitude of ground reaction force at impact is caused by decelerating the downward movement of the body. Another component of ground reaction force is caused by preventing the equal and opposite reaction to the pull of gravity. A way to think about stopping the downward velocity of the body is impulse. mass x velocity = momentum so there is a downward momentum. To change a momentum you need an impulse, which is a force x time. You can have a higher force for a shorter time vs a lower force for a longer time. Movement of the joints makes it so that the trunk stops over a longer period of time and hence lower forces. Now you have to add in the "movement" of the ground as well. (sand vs concrete)


    So, I see an equinous not effecting stiffness at contact as long as the subject is still a heel striker. You can think of the joints of the lower extremity as springs with variable stiffness. The degree of stiffness is going to be determined by the amount of muscular activation and the geometry of the external and internal forces relative to the joints.

    Cheers,

    Eric
     
  4. Ive gone thru my saved papers on and around this subject. Thought I would load them up, some have already been posted in other threads others not .
     
  5. And these
     

    Attached Files:

  6. Petcu Daniel

    Petcu Daniel Well-Known Member

    Hi,

    How this CNS controlled stiffness will function in the case of a foot with reduced
    or loss of protective sensation ?

    Thank you,
    Daniel
     
  7. I guess not so well, especially if the initial GRF point is at a point of the foot where there is a loss of sensation.

    Loss of sensation = Loss of GRF feedback = Loss of leg stiffness regulation
     
  8. From the "plantar feedback" paper me colleague attached above, we see reduction in leg stiffness with tibial nerve block. I don't think this is the same as "loss of leg stiffness regulation" We can still regulate leg stiffness, we just don't have the sensory input from the plantar foot.
     
  9. I´m been trying to track down this article with no luck. Be great if someone can post the full paper or PM me.

     
  10. Griff

    Griff Moderator

    Here you go Mike
     

    Attached Files:

  11. Thats a sit down in a very quite Room type read.
     
  12. Petcu Daniel

    Petcu Daniel Well-Known Member

    - "The Myth of Running Shoe Cushioning", M.R.Shorten - "A closer examination of the ground reaction force reveals factors that may limit it’s value as a measure of cushioning."
    http://www.biomechanica.com/docs/publications/docs/Shorten - The Myth of Running Shoe Cushioning.pdf

    - "Calculation of vertical ground reaction force estimates during running from positional data." Bobbert MF, Schamhardt HC, Nigg BM. - "It follows that if the possible relationship between 'impact force peaks' and injuries is to be investigated, or if the effects of running shoe and surface construction on these force peaks are to be evaluated, the calculation of segmental contributions to Fz(t) is a more suitable approach than measuring only Fz(t).
    http://www.move.vu.nl/wp-content/uploads/2008/02/BobSchNig_1991.pdf
     
  13. efuller

    efuller MVP

    That's a very interesting question. It depends on how much of "stiffness control" is from whold body acceleration (keeping the eyes steady) vs foot inpact. Our bodies are have many redundant systems. If the important thing is to keep the eyes steady then someone with neuropathy in the foot would still have to learn to use other stimuli to keep the eyes steady.

    Regards,

    Eric
     
  14. Having thought about what Simon said in a post above that makes sense.

    same as for people who lose eye sight the other sense take over they say.
     
  15. Asher

    Asher Well-Known Member

    Really enjoying this discussion.

    I can understand that its the instant of heel contact that is important in the body's regulation of leg stiffness, as discussed by Eric. Simon mentions that this is depicted by the gradient of the first up-slope on the force/time curve.

    But I have also read about the heel strike transient which is the little blimp on the way up to the first peak on the force/time curve. It is referred to as the 'rate of loading'. I've noticed that FScan in-shoe doesn't show a heel strike transient.

    Is it the gradient of the heel strike transient or of the entire first up-slope of the force/time curve that is relevant to determining leg stiffness?

    What's the deal?

    Rebecca
     

    Attached Files:

  16. Rebecca my money is on the gradient of the first up slope since we don't have a heel strike transient in midfoot or forefoot striking, but we still see modification of leg stiffness.

    The point Irene Davis made about the heel strike transient in the barefoot running debate thread, was that the transient may be significant in pathology.

    I essence its probably both!
     
  17. ie how do we use the knowledge of leg stiffness to one determine our patient leg stiffness, decide what material and shape of device and then re-test to determine if there has been an appropriate change in leg stiffness as part of our treatment plan.

    Rebecca have you read the paper inpost 52 that Ian put up. The reason that I asked to look at at was that there are 5 methods to determine leg stiffness, I was hoping to find one that could be used in a clinical setting. To be honest I need some ´quite´time to read it and understand- I may never it does seem pretty complex from my quick scan.

    The loading rate question ,I´ve no idea about f-scan stuff, but I´m thinking it ( the loading rate) maybe one of the determining factors in leg stiffness, along with muscle fatigue, muscle strength, hip,knee and ankle stiffness, maybe stj axis position may play a role etc

    and again I´m not sure but we may need many small pieces of the puzzel to determine leg stiffness during gait not just 1

    not sure if that helps, I agree that it would be good to be able to determine leg stiffness in a clinical setting
     
  18. Another question that has popped up in my thinking.

    We have discussed the initial point of contact being very important to the bodies regulation of leg stiffness.

    By using low dye tape or an orthotic is part of what we are doing is getting more surface area involved quickly which allows for more feedback ?
     
  19. Here's a little thought experiment that Dave Smith and I have been thinking about:

    Lets take our mass spring model and make it physical.

    Lets say we have a set of bathroom scales, on this we sit a compression spring (spring); on top of the spring we place a large ball bearing (mass). Now we push down on the spring and ball bearing and release it so that the spring begins to oscillate with ball bearing riding on top. The reading on the bathroom scales will rise as the spring compresses, reaching it's peak as the ball bearing reaches its lowest point; the reading on the scale will fall as the spring extends reaching its lowest reading when the ball bearing is at its highest point.

    So, if we wanted to keep the reading on the scales constant for a period of time, how could we achieve this?
     
  20. Using my car suspension idea in my head

    as the weight ( COM ) is pushed down, the spring would be regulated to become less stiff, so that it takes the weight without an increase in the weight seen on the scale. As the spring begins to expand and the mass goes up it will also then begin to become stiffer so that the weight seen on the scale remains the same.

    What I see is that the spring is controlling the amount of acceleration in the
    F= M*A law. it does this bu adjusting the level of spring stiffness.
     
  21. You are on my wave-length, Mike. We could also stop the spring mass oscillating- right?

    So, some time ago, Rebecca asked about flat spots in the force / time curve- where the force remains relatively constant for a period of time. It is my contention that flat spots occur due to a change in the leg stiffness. And how does the limb regulate stiffness......
     
  22. We could stop the spring oscialling I beleive

    The limb regulates stiffness thru joint motion ie knee flexion/extension.

    so these flat spots are when leg stiffness regulation is equal to the force from foot to foot interface ? A point of equilibrium if you will.

    So just like STJ axis theory we will having many points of equilibrium when forces are in balance and these are the flat spots in the force/time curve ?
     
  23. That's what I think. i.e., the CoM is neither accelerating toward or away from the force plate. We see similar periods of equilibrium at either end of the oscillation but they are much shorter in duration. If we had a graph with very small time intervals, these points should appear as flat spot too.
     
  24. This interpretation of flat spots also has implications for the energetics of the system regarding the transfer of potential and kinetic energy.
     
  25. Sort of off a little what we are discussing, but I found this in a thread newsbot had started.

    Should give the Chiro world something to think about or the one I´ve come across who always talk about the importance Soft orthotics.

    But as we have discussed- soft orthotic increased leg stiffness potential for increased shock to the back.

    This thread is a good brain workout
     
  26. So a flat spot would mean no energy transfer and if we take propulsion when there should be energy return from the achilles ( the spring or catapult effect). If at this point of time if the leg stiffness is in balance there maybe reduced forward momentium ? Change in gait and use of other muscle in compensation ?
     
  27. Griff

    Griff Moderator

    Attached for you Mike. I agree - tis a good workout. I just sometimes feel like I'm trying to bench 200kg before warming up...
     

    Attached Files:

  28. Thanks Ian for the paper,

    I hear ya!
     
  29. Should also make a difference as to whether the flat spot occurs during downward or upward movement of the CoM, i.e. elastic energy storeage or return.
     
  30. Which would also be a factor in muscle fatigue ie too many flat spots or over a long periods less energy store and return increased reliance on muscle contraction for locamotion.
     
  31. OK, so why would the leg need to change stiffness midway through a cycle when I think we are all agreed it would be better not to? Run out of range of motion (RoM), avoidance (physical object, pain)?
     
  32. Now thats a good question, could it also have something to do with the balance between internal and external moments acting on the joint(s) ?

    Edit : I´m not sure the above makes sense. As GRF increases under the FF the knee will have greater external extension moment at the knee , this will mean a new Foot interface feedback position, so maybe it´s the time it takes from the CNS to get more or less muscle function to regulate the leg stiffness.

    and thinking a little more the run of ROM idea makes more and more sense. Ie if we say that the knee is the 1st joint used to regulate leg stiffness, but if the knee joint runs out of ROM the body then moves to the hip or ankle. Different muscles etc this might take some time and a flat spot maybe seen.
     
  33. Perhaps it's all about stiffness! If the joint cannot get any stiffer or any more compliant!
     
  34. Perhaps.

    I think I need a little digest time.

    off to start the weekend. Hope you have a good one Simon, we have had no internet at home, might be fixed by now but be back on Monday if not.

    mike


    ps the person this is for will understand.....( I think a certain Podiatrist from Adelaide when they read this tomorrow and will smile )
     
  35. Cheers Mike have a good one, it's been great talking with you and the others in this thread.

    P.S. If a joint can't get any stiffer- it's at end RoM ;)
     
  36. Petcu Daniel

    Petcu Daniel Well-Known Member

    Hi,

    I have some questions !!
    First :
    I'm a little confused about the terminology : it is right to think at two type of leg stiffness ?
    1- " leg stiffness" as the capacity of the body to accommodate changes in surface stiffness, let say - "functional leg stiffness"
    2- " leg stiffness" as a measure of a pathology, measured through joint/muscle stiffness, let say - "pathological leg stiffness"
    If that classification is a little bit correct it seems to me that in this thread it was discussed mainly the first aspect of the leg stiffness - the "functional" aspect. So, in what measure could influence one type of the leg stiffness the other type ?
    Second :
    If there are the same meaning of "leg stiffness" from the articles : "Lower extremity joint stiffness in runner with low back pain" -J.Hamill and "Running in the real world adjusting leg stiffness for different surfaces"-D.Ferris, could I conclude that it's possible that low back pain to be potential present in a significant percent [!?] of runners which accommodate through increase leg stiffness because of reduction in surface stiffness ? [based on conclusion from the first article and results from the figure 2 from the second article]
    Third :
    "Leg stiffness increased by as much as 68% between the most stiff surface and the least stiff surface" -Ferris' article- The speed was 5 m/s [ and 3,8 m/s in the Hamill's article].
    There are any results for the leg stiffness in walking ? Could be a low level of the adjusting percent of the leg stiffness in walking , without significant influence in selecting the orthoses material [at least hypothetical]?

    Respectfully,
    Daniel
     
  37. Hey internet works.

    Daniel I think that we 1st need to discuss the perfect case 1st, get an understanding of the who,what,why, hows 1st. ie what you discribe as functional leg stiffness or what I think maybe better discribed as Effective Leg Stiffness Regulation or ELSR

    (ANYONE GOT ANYTHING BETTER ?) With ELSR we will keep the leg in the Zones of optimal leg stiffness ie Simons ZOOLS

    Once we get our heads around this process we may better understand the Ineffective Leg Stiffness Regulation ie ILSR which may mean the leg may not be in the ZOOLS at certain times which may lead to pathology.



    I went looking for walking specific papers on stifness but did not find any, Simon said that he remembers that speed does not effect stiffness- I think it´s in 1 of the papers posted.

    But it maybe easier for the body to regulate leg stiffness due to the double support inwalking as oppossed to running.
     
  38. Maybe it would have been quiter around here if the computer could not log on.

    Ive taken this from the Barefoot thread .............

    Full paper is on post 14 of this thread.

    So the question is why does a High Arch foot in this paper have a higher leg stiffness ?

    Maybe Craig has the answer... Weak intrinsic muscles of the foot lead to a higher arch-
    these weaker muscle may mean greater lengthening when under load which may mean the body feels like its in a cushioned shoe-- increased leg stiffness.
     
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